Fractal analyses of some natural systems

Fractal dimensions are estimated by the box-counting method for real world data sets and for mathematical models of three natural systems. 1 he natural systems are nearshore sea wave profiles, the topography of Shei-pa National Park in Taiwan, and the normalised difference vegetation index (NDV1) image of a fresh fern. I he mathematical models which represent the natural systems utilise multi-frequency sinusoids for the sea waves, a synthetic digital elevation model constructed by the mid-point displacement method for the topography and the Iterated Function System (IFS) codes for the fern leaf. The results show that similar fractal dimensions are obtained for discrete sub-sections of the real and synthetic one-dimensional wave data, whilst different fractal dimensions are obtained for discrete sections of the real and synthetic topographical and fern data. The similarities and differences are interpreted in the context of system evolution which was introduced by Mandelbrot (1977). Finally, the results for the fern images show that use of fractal dimensions can successfully separate void and filled elements of the two-dimensional series

Laboratory scale modelling of a point absorber wave energy converter

Finding an effective Power Take Off (PTO) system, that can deal with the low power levels (< 1W), associated with laboratory scale testing of Wave Energy Converts (WEC) is a notoriously difficult problem. This thesis investigates the use a magnetic particle brake to address the issue. The work describes 1:50 scale tests of a generic point absorber device, restricted to heave motions only. The model set-up is not intended to replicate any existing WEC device but rather to provide a good case study for examining performance characteristics such as exciting forces, response amplitudes and power absorption for both regular and irregular wave conditions. The PTO was found to be an effective low cost solution for the scale of operations considered. The brake adequately met the requirements of supplying resistive damping at the typically low speeds and could supply enough force to hold device at the stillwater level for wave force excitation tests. The characteristics of the PTO proved to be ineffective to study the advanced control technique known as latching. A new control method based on Wavelet analysis, utilising the PTOs ability to vary the damping in real time is described. The method yielded marginally improved energy capture. Three different shapes of float have been tested: simple right cylinder, a right cylinder with conical and hemispherical ends. Experimental testing was carried out using the (25m long, 18m wide and 1m deep) wave basin at the Hydraulics & Maritime Research Centre. The cone shape was found to be the most efficient with the use of mechanically induced control forces. A numerical model predicts the theoretical response of the buoy using linear theory, allowing for a validation study between the experimental and numerical results. A time domain model has been developed using Simulink and the boundary element method package WAMIT enabling non-linearity’s in the set-up to be examined. It was found that using a linear hydrostatic restoring force underestimated the resonance frequency of the cone and hemisphere. The numerical models yielded poor prediction results due the increased device motions, violating the linear assumptions

5 European & African Conference on Wind Engineering

The 5th European-African Conference of Wind Engineering is hosted in Florence, Tuscany, the city and the region where, in the early 15th century, pioneers moved the first steps, laying down the foundation stones of Mechanics and Applied Sciences (including fluid mechanics). These origins are well reflected by the astonishing visionary and revolutionary studies of Leonardo Da Vinci, whose kaleidoscopic genius intended the human being to become able to fly even 500 years ago… This is why the Organising Committee has decided to pay tribute to such a Genius by choosing Leonardo's "flying sphere" as the brand of 5th EACWE